In animals, oocyte development arrests at the first meiotic prophase.
Oocyte maturation then resumes in response to specific triggers, which, in
C. elegans, take the form of the major sperm protein (MSP, an ephrin
signalling antagonist). Now, two studies by David Greenstein and co-workers
shed light on how MSP triggers oocyte maturation, and identify crucial roles
for Notch signalling and Gαs-adenylate cyclase (GαAC)
in both oocyte growth and maturation.

In their first study (see
p. 2211), these
researchers report that all germline MSP-dependent oocyte meiotic maturation
events require GαAC signalling in the somatic sheath cells of the gonad,
but not in the germ cells themselves. They demonstrate that the MSP-mediated
events that occur in the germline, including MPK-1 MAP kinase activation,
which triggers meiotic maturation, and cortical microtubule reorganisation,
which probably promotes meiotic spindle assembly, depend on GαAC
signalling. Gap junctions appear to function downstream of GαAC
signalling, as mutations in gap junction protein-encoding genes suppress
GαAC-associated defects. GαAC signalling also promotes
MSP-dependent oocyte growth and cytoplasmic streaming.

In their second study (on
p. 2223), these
investigators go on to show that GLP-1/Notch signalling in the somatic distal
tip cells (DTCs) regulates MSP-dependent oocyte growth and cytoplasmic
streaming. Mutations in glp-1 lead to large oocytes, and, by using
genetic mosaic analysis, in which the fates of clonally derived cells that are
genetically distinct from the surrounding tissue are traced, the authors show
that germline glp-1 regulates normal oocyte growth. The DTC-expressed
GLP-1 ligands LAG-2 and APX-1 probably mediate this regulation, as DTC
ablation or LAG-2 and APX-1 co-depletion result in enlarged oocytes. Moreover,
glp-1 mutations lead to higher rates of cytoplasmic streaming and to
delayed oocyte cellularisation, as long as certain gap junction proteins and
core apoptotic pathways are functional. These results indicate that
GLP-1/Notch signalling restricts oocytes to their normal size in the presence
of MSP signalling.

Taken together, these findings suggest that two major signalling centres in
the adult C. elegans hermaphrodite gonad - GLP-1/Notch distally and
MSP proximally - function in opposition to regulate meiotic maturation and
oocyte growth, a regulatory interaction that might be conserved in other
species.

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